(1) Field of the Invention
This invention relates to semiconductor field-effect transistors, in particular to modulation doped field effect transistors (MODFETs)
(2) Description of the Related Art
In semiconductor device technology, it is desirable to have transistors with high frequency capability. The performance of a conventional metal-oxide-semiconductor (MOS) field-effect transistor (FET) is limited by scattering mechanisms. Scattering is due to the collision of charge carriers (electrons or holes) with impurities, lattice defects, and interface roughness at the SiO2/Si boundary. Dopant impurities are necessary to control the semiconductor conductivity. The technique of modulation doping has been devised to spatially separate the impurities from the charge transport channel. Modulation doped FET (MODFET) structures are known to overcome the scattering problem of a conventional FET.
In a basic semiconductor MODFET structure as shown in FIG. 1, a lightly doped pxe2x88x92InGaAs layer 11 is grown over an InP substrate 10 and is covered by an undoped [or not intentionally doped (NID)] InAlAs spacer layer 12 and an n-type InAlAs supply layer 13. Source contact 14 and drain contact 15 are formed respectively at the two ends of the supply layer 13 in the longitudinal (z) direction on an n+ region 16. A gate 17 of length L and width W covers the channel between the source 14 and drain 15 to control the current flow between them (without making an electrical contact). The gate is typically a Schottky contact on top of a cap layer 18. The n-InGaAs cap layer is grown over the supply layer.
The principle of operation is to establish an electron gas 19 at the heterointerface between the InGaAs layer 11 and the InAlAs spacer 12. The gas 19 forms on the InGaAs layer 11 side of this interface in a triangular conduction band quantum well. Since in this pxe2x88x92 or undoped (NID) layer 11, there are no impurities to scatter the electrons, the electrons can move with reduced collision frequency under an applied electric field between the source 14 and drain 15. As a result of this type of carrier channel, the MODFET has a much better high frequency performance than the order of 150 angstroms or less, then the electron gas 19 is confined in the lateral (x) direction, resulting in a quantum wire MODFET.
The MODFET, however, has a trade-off between impurity scattering and charge control. The spacer layer 12 is included in a typical device to provide a buffer between the electron gas 19 channel and the InAlAs supply layer 13. A thicker spacer reduces the impurity scattering, but at the expense of gate control, which results in a lower device transconductance, and poorer high-frequency performance. Thus, improvements to the basic MODFET structure are desirable.
An object of the present invention is to improve the high frequency performance of FETs and MODFETs. This objective is achieved by inserting a coupled-well transport channel in place of a conventional channel. In the case of MODFETs, this can be achieved by inserting an InGaAs/InAlAs/InGaAs coupled-well layer 11 in MODFETs. The coupled-well is created by two thin layers (wells) of InGaAs with a thin InAlAs barrier separating them. The coupled-well provides a potential profile in the transverse y-direction to move the electron gas 19 away from the interface (between the spacer 12 and the pxe2x88x92InGaAs layer 12), while maintaining the same gate control. For Si/Ge MODFETS, the coupled-well structure may be incorporated in devices for the Schottky type as well as MOS type gates. MOS gate MODFETs permit self-aligned gate structure as in MOSFETs.
For Si MOSFETs, one can have a SiGe/Si/SiGe coupled well adjacent to SiO2xe2x80x94Si (thin) layer. Here, Si layer adjacent to SiO2 gate oxide is thin, and transfers inversion layer charge to one of the SiGe quantum wells in the asymmetric coupled-well configuration. It may be mentioned that metal gate is generally replaced by a poly Si gate.
Another object of this invention is to reduce the width, W, of the channel in the lateral x-direction, so as to realize quantum wire MODFETs, for the purpose of achieving low-power operation, while enhancing the high-frequency performance. A variation of this is to obtain quantum dot MODFETs by reducing the channel length L in the z-direction.
Still another object of this invention is to provide a multiple channel quantum wire MODFET capable of coherent switching and other functions that are not achievable in conventional MODFETs. Still another objective of this invention is to realize a MODFET in which the supply layer is contacted independently, resulting in multiple gates. This enables dynamic selection of operational mode between normally ON (enhancement mode) and normally OFF (depletion mode), resulting in complementary logic. Supply layer contacts also allow precise control of depletion in the supply layer. These types of MODFET structures have an application in heterostructure acoustic charge transport spatial light modulators (HACT-SLMs). Unlike in conventional HACTs/HACT-SLMs, where the depletion is set by the cap layer during processing, and can degrade over time, the current invention allows continual depletion control through biasing.
The multiple channel quantum wire MODFET objective is achieved by laying side by side two laterally (x direction) coupled channels each with individual source, drain, and gate contacts.